In recent years, there have been proposed a plurality of call services in which frequency bands containing transmitted voice signals are different from one another. In a wireless communication system compatible with, for example, Long Term Evolution (LTE), there has been proposed Voice over LTE (VoLTE) in which a communication line compliant with the LTE is utilized and a voice signal is transmitted on an internet protocol (IP) network, thereby realizing a voice call. In the VoLTE, the bandwidth of a transmitted voice signal is set to, for example, about 0 Hz to about 8 kHz and is wider than the bandwidth (about 0 Hz to about 4 kHz) of a voice signal transmitted in a 3G network. Therefore, in a mobile phone in which voice communication services of both the VoLTE and the 3G are provided, in some cases a change in a communication environment or the like causes a communication method for a voice signal to be switched from the VoLTE to the 3G during a voice call. In such a case, since the quality of a received voice changes in association with the switching, a user has a feeling of uncomfortable toward the received voice at the time of the switching in some cases.
Therefore, there has been studied a technology for suppressing discontinuity of a voice signal when the bandwidth of the transmitted voice signal is switched based on a communication environment or the like (see, for example, International Publication Pamphlet No. WO 2006/075663).
To switch the bandwidth of a voice signal to be output, a voice switching device disclosed in, for example, International Publication Pamphlet No. WO 2006/075663, outputs a mixed signal in which a narrowband voice signal and a wideband voice signal are mixed. In addition, this voice switching device changes, with time, a mixing ratio between the narrowband voice signal and the wideband voice signal.